Method and device for recovering phosphorus in waste water by micro-bubble assisted struvite secondary crystallization

Abstract

The application discloses a method and device for recovering phosphorus in waste water by using micro-bubble assisted struvite secondary crystallization, which comprises the following steps: firstly, primary crystallization of struvite is completed by using a static mixer to obtain struvite microcrystals, and conditions for secondary crystallization are created; and then, secondary crystallization of struvite is completed by using micro-bubbles generated by dissolved air flotation in a gas floating crystallizer, and finally, struvite crystallization products with controllable particle size and excellent sedimentation performance are obtained, and the recovery effect of phosphorus in waste water can be improved.

Description

Technical Field

[0001] This invention belongs to the field of phosphorus resource utilization technology in wastewater. Specifically, this invention relates to a method and apparatus for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles. Background Technology

[0002] Struvite (MgNH4PO4·6H2O) crystallization is a promising phosphorus recovery technology. However, in traditional struvite crystallization phosphorus recovery processes, although the phosphorus crystallization rate is very high, the recovery rate is very limited. This is mainly because when struvite crystallization adopts a homogeneous crystallization mode, it easily generates a large number of fine crystal particles (<50 μm), which have extremely poor settling properties, leading to difficulties in solid-liquid separation and severely weakening the phosphorus recovery effect. While introducing seed crystals into the struvite crystallization system to regulate homogeneous crystallization into induced crystallization can obtain crystallized products with controllable particle size, thereby improving the phosphorus recovery rate, induced crystallization requires a fluidized bed reactor. However, the seed crystals are in a fluidized state in the fluidized bed and easily flow out of the fluidized bed with the rising water flow. The continuous loss of seed crystals gradually weakens the phosphorus recovery effect of struvite crystallization. Solving the problem of seed crystal loss with the water flow requires a complex and costly seed crystal recovery and reflux system, which limits the engineering application of struvite induced crystallization phosphorus recovery.

[0003] Moreover, when PO4 in wastewater 3- When the concentration is high, the supersaturation of the crystallization system is also relatively high. At this time, even if seed crystals are added to the struvite crystallization system, homogeneous crystallization is still inevitable. Moreover, the higher the supersaturation of the crystallization system, the more dominant homogeneous crystallization becomes, and a large number of fine crystal particles with poor settling performance will be generated, reducing the phosphorus recovery rate.

[0004] Therefore, to achieve efficient and stable phosphorus recovery from wastewater through struvite crystallization, the corresponding technology and equipment must possess both high crystallization efficiency and excellent solid-liquid separation performance of the crystallized products. To date, there are no publicly reported methods or equipment for phosphorus recovery via struvite crystallization that possess these properties. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of poor particle settling performance in the homogeneous crystallization mode and low recovery rate in the induced crystallization mode due to easy loss of crystal seeds during the phosphorus recovery process of struvite crystallization in phosphorus-containing wastewater. The invention provides a method and device for secondary crystallization of struvite assisted by microbubbles generated by dissolved air flotation, so as to achieve controllable particle size of crystallized products, excellent settling performance and high phosphorus recovery rate.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, embodiments of the present invention provide a method for recovering phosphorus from wastewater using microbubble-assisted secondary crystallization of struvite, comprising the following steps: S1: After adjusting the pH of the phosphorus-containing wastewater to alkaline, magnesium salt and ammonium salt are added to the phosphorus-containing wastewater. The resulting mixture is then transported to a static mixer for a primary crystallization reaction of struvite to obtain phosphorus-containing wastewater containing struvite microcrystals. S2: The phosphorus-containing wastewater containing struvite microcrystals is pumped into an air flotation crystallizer equipped with a high-pressure dissolved air release mechanism at the bottom. After flowing downward through the guide zone of the air flotation crystallizer, it flows horizontally into the air flotation zone of the air flotation crystallizer through a perforated plate. The struvite microcrystals in the phosphorus-containing wastewater in the air flotation zone agglomerate with the microbubbles in the air flotation crystallizer to form agglomerates and float to the upper middle part of the air flotation crystallizer to form a suspended sludge zone. The residual wastewater meets the discharge standards. S3: The struvite microcrystals in the suspended sludge zone undergo secondary crystallization and maturation under mechanical stirring until their particle size reaches the threshold. Then, under gravity, they settle to the sedimentation zone at the bottom of the air flotation crystallizer to achieve solid-liquid separation. The settled struvite crystal particles are scraped into the sludge tank by a scraper to achieve phosphorus recovery.

[0007] In some embodiments, the specific process of step S3 is as follows: S31: Due to the rupture of microbubbles, the aggregates in the suspended sludge zone break down, releasing struvite microcrystals that disperse throughout the suspended sludge zone. S32: Under the stirring action of the stirring mechanism in the suspended sludge zone, the dispersed struvite microcrystals in the suspended sludge zone collide with the existing struvite crystal particles in the suspended sludge zone at a high frequency. Some of the struvite microcrystals agglomerate with the struvite crystal particles, achieving direct secondary crystallization and generating struvite crystal particles with larger particle sizes. The other part of the struvite microcrystals that fail to agglomerate with the struvite crystal particles dissolves under the stirring action, releasing crystal-forming ions. The crystal-forming ions undergo indirect secondary crystallization on the surface of the struvite crystal particles. S33: Under the action of direct secondary crystallization and indirect secondary crystallization in step S32, the particle size of struvite crystals in the suspended sludge zone gradually increases. When it increases to a certain threshold, the air flotation effect of microbubbles is insufficient to overcome the gravity of the struvite crystals, causing the struvite crystals to settle to the bottom sedimentation zone of the air flotation crystallizer under gravity, thus achieving solid-liquid separation. The settled struvite crystals are then scraped into the sludge tank by a scraper to achieve phosphorus recovery.

[0008] In some embodiments, in step S1, the pH value of the phosphorus-containing wastewater is adjusted to 9.0~10.5; And / or, the magnesium salt is magnesium chloride or magnesium sulfate, and the ammonium salt is ammonium chloride or ammonium sulfate; And / or, the Mg in the magnesium salt 2+ With the NH4 in the ammonium salt + PO4 in the phosphorus-containing wastewater 3- The molar ratio is (1~1.1):(1~1.3):1; And / or, the time for the primary crystallization reaction is 10~30 s.

[0009] In some embodiments, in step S2, the dissolved gas pressure generated by the high-pressure dissolved gas water release mechanism is 0.5 MPa.

[0010] In some embodiments, in step S2, the air-to-water volume ratio of the air flotation zone is ≥10%.

[0011] Secondly, embodiments of the present invention also propose a device for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles. This device is used to implement the method described in the first aspect and includes an alkali storage tank, a magnesium salt storage tank, an ammonium salt storage tank, a static mixer, an air flotation crystallizer, and a pressure dissolved air tank. The air flotation crystallizer includes a flow guiding zone and a reaction zone from left to right. The reaction zone includes a suspended sludge zone, an air flotation zone, and a sediment zone connected sequentially from top to bottom. A high-pressure dissolved air water release mechanism is provided at the bottom of the air flotation crystallizer. This high-pressure dissolved air water release mechanism is composed of several parallel perforated pipes. The inlet of the high-pressure dissolved air water release mechanism is connected to the outlet of the pressure dissolved air tank. The outlet of the alkali storage tank is connected to the inlet of the static mixer, the outlet of the magnesium salt storage tank is connected to the inlet of the static mixer, the outlet of the ammonium salt storage tank is connected to the inlet of the static mixer, and the outlet of the static mixer is connected to the inlet of the flow guiding zone of the air flotation crystallizer.

[0012] In some embodiments, the hydraulic residence time of the diversion zone is 10-20 s; And / or, the flow guiding area and the reaction area are separated by a perforated plate.

[0013] In some embodiments, the height ratio of the air flotation zone to the suspended sludge zone is (1~1.5):1, and the length-to-height ratio of the air flotation zone is not less than 4.

[0014] In some embodiments, a stirring mechanism is provided in the suspended sludge zone, and the stirring speed of the stirring mechanism is not less than 100 rpm.

[0015] In some embodiments, a slag scraper and a sludge trough are provided at the bottom of the sedimentation zone, and the sludge trough is inclinedly connected to one side wall of the air flotation crystallizer.

[0016] The advantages and beneficial effects of the embodiments of the present invention are as follows: (1) The crystallization of struvite is controlled at the microcrystal formation stage by using a static mixer, which creates conditions for the efficient secondary crystallization of microcrystals in the future.

[0017] (2) By utilizing the continuous air flotation top support effect of microbubbles, a stable suspended sludge zone is formed, realizing the separation of the hydraulic residence time of the device and the crystallization reaction time of struvite, ensuring that struvite microcrystals have enough time for secondary crystallization.

[0018] (3) The crystallization rate of struvite crystals is significantly improved by primary crystallization in the static mixer and secondary crystallization in the suspended sludge zone; and the particle size of struvite crystal products is increased by secondary crystallization in the suspended sludge zone, thereby improving its settling performance and solid-liquid separation ability, and greatly improving the recovery rate of struvite crystals.

[0019] (4) During the crystallization and recycling process, no other chemical agents need to be added except for the struvite crystal particles. The resulting struvite crystal products have high purity and high recycling value.

[0020] (5) By adjusting the air-water ratio in the flotation zone, the particle size of struvite crystal products can be dynamically controlled to meet different resource utilization requirements. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a device for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by an embodiment of the present invention.

[0022] Attached reference numerals: 1-Alkali storage tank, 2-Magnesium salt storage tank, 3-Ammonium salt storage tank, 4-Static mixer, 5-Pressure pump, 6-Air flotation crystallizer, 601-Guiding zone, 602-Perforated plate, 603-Air flotation zone, 604-Suspended sludge zone, 6041-Agitator, 605-Settling zone, 6051-Sludge scraper, 6052-Sludge trough, 606-Perforated pipe, 7-Pressure dissolved air tank, 8-Water pump. Detailed Implementation

[0023] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0024] In this invention, when a value is described as a range, it should be understood that such disclosure includes disclosure of all possible subranges within that range, as well as specific numerical values ​​falling within that range, regardless of whether specific numerical values ​​or specific subranges are explicitly specified.

[0025] In this invention, the terms “comprising” and “including” and their various variations mean that other elements or wholes may be included but are not specifically described.

[0026] In this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0027] This invention is based on the inventor's discoveries and understanding of the following facts and problems: The inventors discovered that when using struvite crystallization to recover phosphorus from wastewater, although the phosphorus crystallization rate is high, the recovery rate is often limited. The main reason for this is that when using a homogeneous crystallization mode, the struvite crystallization nucleation process cannot be effectively controlled, generating a large number of struvite microcrystals that are difficult to settle and separate from the liquid, thus weakening the phosphorus recovery effect. Although extending the crystallization reaction time can allow the struvite microcrystals to mature into larger crystal particles, the required crystallization reaction time often exceeds several days, which significantly exceeds the allowable hydraulic retention time for water treatment (e.g., less than 2 hours).

[0028] While seed-induced crystallization allows for control of the nucleation process and consequently the particle size of the crystallized product, resulting in crystals with excellent settling properties and ensuring stable phosphorus recovery, extensive practical experience shows that induced crystallization requires a fluidized bed reactor. This means that seed crystals are easily lost from the fluidized bed, leading to a rapid decrease in phosphorus recovery. On one hand, as the crystallized product is separated and recovered, seed crystals are continuously discharged from the fluidized bed; on the other hand, the fluidization of seed crystals requires an upward water flow, which can cause seed crystals to easily flow out of the fluidized bed.

[0029] Based on this, the present invention utilizes microbubbles to assist in the secondary crystallization of struvite, thereby obtaining struvite crystal products with controllable particle size and excellent settling performance, which significantly improves the phosphorus recovery effect of phosphorus-containing wastewater. First, a static mixer completes the homogeneous nucleation process of struvite, i.e., primary crystallization, to obtain struvite microcrystals, creating conditions for secondary crystallization of struvite assisted by microbubble flotation. Second, microbubbles released from high-pressure dissolved air water agglomerate with the struvite microcrystals to form aggregates. These aggregates reach the suspended sludge zone via flotation, and under the continuous support of microbubbles, the sludge zone is stably suspended in the upper part of the flotation crystallizer. Third, the aggregates entering the suspended sludge zone release struvite microcrystals due to the rupture of microbubbles. Under the stirring action of the stirring mechanism, these microcrystals undergo direct secondary crystallization by colliding with existing struvite crystal particles in the suspended sludge zone, or indirect secondary crystallization through the Ostwald ripening process of "dissolution-recrystallization". Finally, as secondary crystallization continues, the particle size of the struvite crystals in the suspended sludge zone continuously increases. After reaching a threshold, they settle to the sedimentation zone by gravity and are then discharged through the sludge tank, realizing the resource utilization of phosphorus in phosphorus-containing wastewater.

[0030] In a first aspect, embodiments of the present invention provide a method for recovering phosphorus from wastewater using microbubble-assisted secondary crystallization of struvite, comprising the following steps: S1: After adjusting the pH of the phosphorus-containing wastewater to alkaline, magnesium salt and ammonium salt are added to the phosphorus-containing wastewater. The resulting mixture is then transported to a static mixer for a primary crystallization reaction of struvite to obtain phosphorus-containing wastewater containing struvite microcrystals. S2: The phosphorus-containing wastewater containing struvite microcrystals is pumped into an air flotation crystallizer equipped with a high-pressure dissolved air release mechanism at the bottom. After flowing downward through the guide zone of the air flotation crystallizer, it flows horizontally into the air flotation zone of the air flotation crystallizer through a perforated plate. The struvite microcrystals in the phosphorus-containing wastewater in the air flotation zone agglomerate with the microbubbles in the air flotation crystallizer to form agglomerates and float to the upper middle part of the air flotation crystallizer to form a suspended sludge zone. The residual wastewater meets the discharge standards. S3: The struvite microcrystals in the suspended sludge zone undergo secondary crystallization and maturation under mechanical stirring until their particle size reaches the threshold. Then, under gravity, they settle to the sedimentation zone at the bottom of the air flotation crystallizer to achieve solid-liquid separation. The settled struvite crystal particles are scraped into the sludge tank by a scraper to achieve phosphorus recovery.

[0031] In some embodiments, the specific process of step S3 is as follows: S31: Due to the rupture of microbubbles, the aggregates in the suspended sludge zone break down, releasing struvite microcrystals that disperse throughout the suspended sludge zone. S32: Under the stirring action of the stirring mechanism in the suspended sludge zone, the dispersed struvite microcrystals in the suspended sludge zone collide with the existing struvite crystal particles in the suspended sludge zone at a high frequency. Some of the struvite microcrystals agglomerate with the struvite crystal particles, achieving direct secondary crystallization and generating struvite crystal particles with larger particle sizes. The other part of the struvite microcrystals that fail to agglomerate with the struvite crystal particles dissolves under the stirring action, releasing crystal-forming ions. The crystal-forming ions undergo indirect secondary crystallization on the surface of the struvite crystal particles (i.e., indirect secondary crystallization of "dissolution-recrystallization", also known as the Ostwald ripening process). S33: Under the action of direct secondary crystallization and indirect secondary crystallization in step S32, the particle size of struvite crystals in the suspended sludge zone gradually increases. When it increases to a certain threshold, the air flotation effect of microbubbles is insufficient to overcome the gravity of the struvite crystals, causing the struvite crystals to settle to the bottom sedimentation zone of the air flotation crystallizer under gravity, thus achieving solid-liquid separation. The settled struvite crystals are then scraped into the sludge tank by a scraper to achieve phosphorus recovery.

[0032] In some embodiments, in step S1, the pH value of the phosphorus-containing wastewater is adjusted to 9.0-10.5. The inventors have found that when the pH of the phosphorus-containing wastewater is controlled within the range of 9.0-10.5, not only is the formation rate of struvite microcrystals faster, but the generated struvite microcrystals are also more easily carried into the suspended sludge zone by microbubbles. However, if the pH value of the phosphorus-containing wastewater is below 9.0, it will lead to increased PO4 content. 3- and Mg 2+ and NH4 + The wastewater was sent to the flotation zone before it could crystallize, causing a sharp drop in phosphorus recovery. Furthermore, when the pH of the phosphorus-containing wastewater was above 10.5, it led to an increase in magnesium content. 2+ This will generate Mg(OH)2 crystals; not only will the phosphorus recovery rate be reduced, but the purity of the recovered product will also be reduced due to the mixing of Mg(OH)2. Therefore, it is appropriate to control the pH value of the phosphorus-containing wastewater in the range of 9.0 to 10.5 in the embodiments of the present invention. And / or, the magnesium salt is magnesium chloride or magnesium sulfate, and the ammonium salt is ammonium chloride or ammonium sulfate; And / or, the Mg in the magnesium salt 2+ With the NH4 in the ammonium salt + PO4 in the phosphorus-containing wastewater 3- The molar ratio is (1~1.1):(1~1.3):1; And / or, the time for the primary crystallization reaction is 10~30 s.

[0033] In some embodiments, in step S2, the dissolved gas pressure generated by the high-pressure dissolved gas water release mechanism is 0.5 MPa, which ensures that the diameter of the generated microbubbles is about 30 μm, which is most conducive to the formation of the suspended sludge zone.

[0034] In some embodiments, in step S2, the air-to-water volume ratio in the flotation zone is ≥10%, which ensures that the average particle size of the recovered struvite crystal particles is stable at ≥500 μm, resulting in excellent solid-liquid separation performance. This allows most of the phosphorus in the wastewater to be recovered in the form of struvite crystal particles.

[0035] Secondly, such as Figure 1As shown in the figure, this invention also proposes a device for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles. The device is used to implement the method described in the first aspect and includes an alkaline storage tank 1, a magnesium salt storage tank 2, an ammonium salt storage tank 3, a static mixer 4, an air flotation crystallizer 6, and a pressure dissolved air tank 7. The air flotation crystallizer 6 includes a guide zone 601 and a reaction zone from left to right. The reaction zone includes a suspended sludge zone 604, an air flotation zone 603, and a sedimentation zone 605 connected sequentially from top to bottom. A high-pressure dissolved air water release mechanism is provided at the bottom of the air flotation crystallizer 6. The dissolved air water release mechanism consists of several parallel perforated pipes 606; the inlet of the high-pressure dissolved air water release mechanism is connected to the outlet of the pressure dissolved air tank 7, and a water pump 8 is provided on the connecting pipe; the outlet of the alkali storage tank 1 is connected to the inlet of the static mixer 4, the outlet of the magnesium salt storage tank 2 is connected to the inlet of the static mixer 4, the outlet of the ammonium salt storage tank 3 is connected to the inlet of the static mixer 4, and the outlet of the static mixer 4 is connected to the inlet of the guide zone 601 of the air flotation crystallizer 6, and a pressurizing pump 5 is provided on the connecting pipe.

[0036] It should be noted that the above-mentioned static mixer 4 is composed of one or more parallel static mixing components, and the number of parallel components is not particularly limited. Those skilled in the art can determine the number of components based on the amount of wastewater. The mixing time of the static mixer 4 (i.e., the reaction time of the first crystallization reaction) is controlled to be 10~30 s.

[0037] It should also be noted that the alkaline solution in alkaline storage tank 1 is not particularly limited. Those skilled in the art can store alkaline solution according to actual needs. For example, the alkaline solution in alkaline storage tank 1 can be Ca(OH)2 solution, NaOH solution, etc.

[0038] In some embodiments, the hydraulic retention time of the guide zone 601 is 10-20 s; by setting the guide zone 601, the downward flow of phosphorus-containing wastewater can be adjusted to a stable horizontal flow. And / or, the flow guiding area 601 is separated from the reaction area by a perforated plate 602, and the perforated plate 602 is provided with a plurality of water holes.

[0039] In some embodiments, the height ratio of the air flotation zone 603 to the suspended sludge zone 604 is (1~1.5):1, and the length-to-height ratio of the air flotation zone 603 is not less than 4.

[0040] In some embodiments, a stirring mechanism 6041 is provided in the suspended sludge zone 604, and the stirring speed of the stirring mechanism 6041 is not less than 100 rpm.

[0041] In some embodiments, the bottom of the sedimentation zone 605 is provided with a slag scraper 6051 and a sludge trough 6052, and the sludge trough 6052 is inclinedly connected to one side wall of the air flotation crystallizer 6.

[0042] The working process of the device for recovering phosphorus from wastewater by microbubble-assisted secondary crystallization of struvite according to an embodiment of the present invention is as follows: After adjusting the pH of the phosphorus-containing wastewater with alkaline solution from alkaline storage tank 1, the solution is mixed with magnesium salt from magnesium salt storage tank 2 and ammonium salt from ammonium salt storage tank 3 before entering static mixer 4. In static mixer 4, the PO4 in the phosphorus-containing wastewater is... 3- With NH4 + and Mg 2+ Nucleation reactions occur rapidly, generating struvite microcrystals. Wastewater containing struvite microcrystals is pumped into the guide zone 601 of the air flotation crystallizer 6 by the pressurized pump 5. After flowing through the guide zone 601 in a downward flow form, it flows into the air flotation zone 603 of the air flotation crystallizer 6 in a horizontal flow form through the perforated plate 602. At the same time, high-pressure dissolved air water from the pressure dissolved air tank 7 is pumped into the perforated pipe 606 of the high-pressure dissolved air water release mechanism by the water pump 8. The high-pressure dissolved air water released by the perforated pipe 606 generates a large number of microbubbles, which mix with the wastewater containing struvite microcrystals flowing in from the perforated plate 602. The struvite microcrystals and microbubbles violently agglomerate to form agglomerates, which are then quickly floated to the suspended sludge zone 604. The remaining wastewater is discharged in compliance with standards. The struvite microcrystals and existing struvite crystals gathered in the suspended sludge zone 604 are stabilized in the upper and middle parts of the suspended air flotation crystallizer 6 under the continuous support of microbubbles. Under the stirring action of the stirring mechanism 6041, the struvite microcrystals in the suspended sludge zone 604 complete secondary crystallization. When the average particle size of the struvite crystals in the suspended sludge zone 604 grows to the threshold, gravity will overcome the support of microbubbles. Under the action of gravity, the mature struvite crystal particles settle into the sedimentation zone 605 and are periodically scraped into the sludge tank 6052 by the scraper 6051. After the struvite crystallization products in the sludge tank 6052 are discharged from the air flotation crystallizer 6, they are naturally dried to realize the resource utilization of phosphorus.

[0043] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. Experimental methods without specific conditions in the embodiments are conventional methods and conditions well known in the art.

[0044] Example 1 In this embodiment, the phosphorus-containing wastewater to be recycled and treated is biogas slurry from a pig farm, with a total phosphorus concentration of 85 mg / L, including PO4. 3- The concentration was 78 mg / L, the pH value was 6.2, and the average influent flow rate was 2 m³. 3 / h. The method for recovering phosphorus from this wastewater includes the following steps: S1: Adjust the pH of the biogas slurry from the pig farm to 10.0 using NaOH solution, and then proceed according to Mg... 2+ NH4 + and PO4 3- With a molar ratio of 1:1.1:1, magnesium chloride and ammonium chloride were added to the biogas slurry of the pig farm, and the resulting mixture was transported to a static mixer for a primary crystallization reaction of struvite to obtain phosphorus-containing wastewater containing struvite microcrystals. S2: The phosphorus-containing wastewater containing struvite microcrystals is pumped into an air flotation crystallizer equipped with a high-pressure dissolved air release mechanism at the bottom. After flowing downward through the guide zone of the air flotation crystallizer, it flows horizontally into the air flotation zone of the air flotation crystallizer through a perforated plate. The struvite microcrystals in the phosphorus-containing wastewater in the air flotation zone agglomerate with the microbubbles in the air flotation crystallizer to form agglomerates and float to the middle and upper part of the air flotation crystallizer to form a suspended sludge zone. The residual wastewater meets the discharge standards. S3: In the suspended sludge zone, the agglomerates break down due to the rupture of microbubbles, releasing struvite microcrystals that disperse throughout the zone. Under the stirring action of the agitation mechanism in the suspended sludge zone, these dispersed struvite microcrystals collide with existing struvite crystal particles at a high frequency. Some of these microcrystals agglomerate with the struvite crystal particles, achieving direct secondary crystallization and generating larger struvite crystal particles. The remaining microcrystals that fail to agglomerate with the struvite crystal particles are further agglomerated during the stirring process. Dissolution occurs under stirring, releasing crystal-forming ions. These ions then undergo indirect secondary crystallization on the surface of the struvite crystal particles. Under the combined effects of direct and indirect secondary crystallization, the particle size of the struvite crystals in the suspended sludge zone gradually increases. When this increases to a certain threshold, the flotation effect of the microbubbles is insufficient to overcome the gravity of the struvite crystals, causing them to settle to the bottom sedimentation zone of the flotation crystallizer under gravity, thus achieving solid-liquid separation. The settled struvite crystals are then scraped into the sludge tank by a scraper, enabling phosphorus recovery.

[0045] In this embodiment, the mixing time (i.e., the reaction time of the primary crystallization reaction) in the static mixer is controlled to be 20 seconds; the hydraulic residence time in the flotation zone is controlled to be 10 minutes; the height ratio of the flotation zone to the suspended sludge zone is 1:1; the air-to-water ratio in the flotation zone is 10%; the dissolved air pressure generated by the high-pressure dissolved air release mechanism is 0.5 MPa, and the diameter of the generated microbubbles is about 30 μm; the stirring speed of the stirring mechanism in the suspended sludge zone is 100 rpm; and the sludge scraping cycle in the sedimentation zone is 48 hours.

[0046] Experimental results show that after treatment using the method described in this embodiment, the phosphorus concentration in the effluent is consistently below 3 mg / L, and the phosphorus recovery rate is consistently above 95%. The recovered struvite crystals have an average particle size of 800 μm and a moisture content of less than 40%.

[0047] Example 2 In this embodiment, the phosphorus-containing wastewater to be recycled is the supernatant from the thickening tank of a municipal wastewater treatment plant, with a total phosphorus concentration of 8-12 mg / L, including PO4. 3- The content is over 90%, and the average influent volume is 2 m³. 3 / h.

[0048] The method for recovering phosphorus from wastewater in this embodiment is basically the same as that in Embodiment 1, except that: in this embodiment, Mg 2+ NH4 + and PO4 3- The molar ratio is 1.2:1.3:1; the hydraulic residence time in the air flotation zone is controlled at 15 min; the height ratio of the air flotation zone to the suspended sludge zone is 1.5:1; the stirring speed of the stirring mechanism in the suspended sludge zone is 150 rpm; and the sludge scraping cycle in the sedimentation zone is 60 h.

[0049] Experimental results show that after treatment using the method described in this embodiment, the phosphorus concentration in the effluent is consistently below 0.5 mg / L, and the phosphorus recovery rate is consistently above 95%. The recovered struvite crystals have an average particle size of 800 μm and a moisture content of less than 40%.

[0050] Example 3 In this embodiment, the phosphorus-containing wastewater to be recycled and treated is high-phosphorus wastewater from a phosphate fertilizer plant, with a total phosphorus concentration of 650~800 mg / L, including PO4. 3- The content is over 95%, and the average influent volume is 2 m³. 3 / h.

[0051] The method for recovering phosphorus from wastewater in this embodiment is basically the same as in Embodiment 1, except that: in this embodiment, NaOH solution is used to adjust the pH of the high-phosphorus-concentration wastewater from the phosphate fertilizer plant to 9.0; and Mg 2+ NH4 + and PO4 3- The molar ratio is controlled at 1:1:1; the hydraulic residence time in the air flotation zone is controlled at 5 min; the stirring speed of the stirring mechanism in the suspended sludge zone is 180 rpm; and the sludge scraping cycle in the sedimentation zone is 36 h.

[0052] Experimental results show that after treatment using the method described in this embodiment, the phosphorus concentration in the effluent is consistently below 1 mg / L, and the phosphorus recovery rate is consistently above 99%. The recovered struvite crystals have an average particle size of 1000 μm and a moisture content of less than 40%.

[0053] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0054] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0055] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0056] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0057] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0058] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles, characterized in that, Includes the following steps: S1: After adjusting the pH of the phosphorus-containing wastewater to alkaline, magnesium salt and ammonium salt are added to the phosphorus-containing wastewater. The resulting mixture is then transported to a static mixer for a primary crystallization reaction of struvite to obtain phosphorus-containing wastewater containing struvite microcrystals. S2: The phosphorus-containing wastewater containing struvite microcrystals is pumped into an air flotation crystallizer equipped with a high-pressure dissolved air release mechanism at the bottom. After flowing downward through the guide zone of the air flotation crystallizer, it flows horizontally into the air flotation zone of the air flotation crystallizer through a perforated plate. The struvite microcrystals in the phosphorus-containing wastewater in the air flotation zone agglomerate with the microbubbles in the air flotation crystallizer to form agglomerates and float to the upper middle part of the air flotation crystallizer to form a suspended sludge zone. The residual wastewater meets the discharge standards. S3: The struvite microcrystals in the suspended sludge zone undergo secondary crystallization and maturation under mechanical stirring until their particle size reaches the threshold. Then, under gravity, they settle to the sedimentation zone at the bottom of the air flotation crystallizer to achieve solid-liquid separation. The settled struvite crystal particles are scraped into the sludge tank by a scraper to achieve phosphorus recovery.

2. The method for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles according to claim 1, characterized in that, The specific process of step S3 is as follows: S31: Due to the rupture of microbubbles, the aggregates in the suspended sludge zone break down, releasing struvite microcrystals that disperse throughout the suspended sludge zone. S32: Under the stirring action of the stirring mechanism in the suspended sludge zone, the dispersed struvite microcrystals in the suspended sludge zone collide with the existing struvite crystal particles in the suspended sludge zone at a high frequency. Some of the struvite microcrystals agglomerate with the struvite crystal particles, achieving direct secondary crystallization and generating struvite crystal particles with larger particle sizes. The other part of the struvite microcrystals that fail to agglomerate with the struvite crystal particles dissolves under the stirring action, releasing crystal-forming ions. The crystal-forming ions undergo indirect secondary crystallization on the surface of the struvite crystal particles. S33: Under the action of direct secondary crystallization and indirect secondary crystallization in step S32, the particle size of struvite crystals in the suspended sludge zone gradually increases. When it increases to a certain threshold, the air flotation effect of microbubbles is insufficient to overcome the gravity of the struvite crystals, causing the struvite crystals to settle to the bottom sedimentation zone of the air flotation crystallizer under gravity, thus achieving solid-liquid separation. The settled struvite crystals are then scraped into the sludge tank by a scraper to achieve phosphorus recovery.

3. The method for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles according to claim 1, characterized in that, In step S1, the pH value of the phosphorus-containing wastewater is adjusted to 9.0~10.5; And / or, the magnesium salt is magnesium chloride or magnesium sulfate, and the ammonium salt is ammonium chloride or ammonium sulfate; And / or, the Mg in the magnesium salt 2+ With the NH4 in the ammonium salt + PO4 in the phosphorus-containing wastewater 3- The molar ratio is (1~1.1):(1~1.3):1; And / or, the time for the primary crystallization reaction is 10~30 s.

4. The method for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles according to claim 1, characterized in that, In step S2, the dissolved gas pressure generated by the high-pressure dissolved gas water release mechanism is 0.5 MPa.

5. The method for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles according to claim 1, characterized in that, In step S2, the air-to-water volume ratio in the flotation zone is ≥10%.

6. A device for recovering phosphorus from wastewater by secondary crystallization of struvite assisted by microbubbles, characterized in that, The apparatus is used to implement the method according to any one of claims 1-5, comprising an alkali storage tank, a magnesium salt storage tank, an ammonium salt storage tank, a static mixer, an air flotation crystallizer, and a pressure dissolved air tank; the air flotation crystallizer includes a guide zone and a reaction zone from left to right, the reaction zone including a suspended sludge zone, an air flotation zone, and a sediment zone connected sequentially from top to bottom; a high-pressure dissolved air water release mechanism is provided at the bottom of the air flotation crystallizer, the high-pressure dissolved air water release mechanism being composed of several parallel perforated pipes; the inlet of the high-pressure dissolved air water release mechanism is connected to the outlet of the pressure dissolved air tank; the outlet of the alkali storage tank is connected to the inlet of the static mixer, the outlet of the magnesium salt storage tank is connected to the inlet of the static mixer, the outlet of the ammonium salt storage tank is connected to the inlet of the static mixer, and the outlet of the static mixer is connected to the inlet of the guide zone of the air flotation crystallizer.

7. The device for recovering phosphorus from wastewater by secondary crystallization of struvite according to claim 6, characterized in that, The hydraulic residence time in the diversion zone is 10~20 s; And / or, the flow guiding area and the reaction area are separated by a perforated plate.

8. The device for recovering phosphorus from wastewater by secondary crystallization of struvite according to claim 6, characterized in that, The height ratio of the air flotation zone to the suspended sludge zone is (1~1.5):1, and the length-to-height ratio of the air flotation zone is not less than 4.

9. The device for recovering phosphorus from wastewater by secondary crystallization of struvite assisted according to claim 6, characterized in that, The suspended sludge zone is equipped with a stirring mechanism, and the stirring speed of the stirring mechanism is not less than 100 rpm.

10. The device for recovering phosphorus from wastewater by secondary crystallization of struvite according to claim 6, characterized in that, The bottom of the sedimentation zone is equipped with a slag scraper and a sludge trough, and the sludge trough is inclinedly connected to one side wall of the air flotation crystallizer.

Images